Weighing the Universe

The universe, astronomers tell us, is expanding. But will it continue to do so forever, or does the universe have enough mass so that its own gravity will eventually slow it down and perhaps even make it collapse into a dense fireball? Because the mass of the universe can’t be measured directly, astronomers have been unable to choose among competing theoretical models. By 2001, though, astronomers may finally have an answer to what may be the ultimate question, thanks to a $70 million mission-- approved by nasa in April--that will map the background radiation left over from the Big Bang in unprecedented detail.

Astronomers got their first close look at the cosmic microwave background in 1992 from data collected by cobe, the Cosmic Background Explorer satellite. cobe revealed small temperature fluctuations in the microwave background. These arose, astronomers believe, as energy unleashed by the Big Bang dissipated, allowing freewheeling, high-energy electrons and protons that formed shortly after the primordial explosion to slow down enough to form atoms of matter. Different regions of space contained varying amounts of matter and thus absorbed and reradiated energy differently, producing the temperature wrinkles that cobe measured.

Important as its findings were, cobe still did not have the resolution to give astronomers the information they needed to measure the density--and destiny--of the universe. cobe’s main objective was to determine whether the fluctuations were there at all, says Chuck Bennett, who is leading the new nasa project. cobe’s resolution allowed it to examine the sky in patches roughly as wide as 14 full moons.

NASA’s new spacecraft, the Microwave Anisotropy Probe, or map, equipped with state-of-the-art electronics, will gather 100 times as much data as cobe. And while some ground-based telescopes now being developed will also improve on cobe--they may be able to spot galaxies being born in the early universe, for instance--Earth’s atmosphere will still limit their view of the microwave background in such a way as to make them less suitable for measuring the overall density of the universe. Once map’s data are in, says Bennett, astronomers will be able to run computer simulations using different values for the cosmic density and then see which model most closely reproduces the microwave fluctuations measured by map. The density used in the successful simulation will then tell astronomers whether the universe has enough matter to halt its expansion.

We’ll need six months to cover the sky, and a certain amount of time to analyze the data, says Bennett, but by 2001 we will know a lot of answers. It’s pretty phenomenal.